GB2048499A

GB2048499A - Device for displaying a signal at a selected frequency in a band

Info

Publication number: GB2048499A
Application number: GB8011391A
Authority: GB
Original assignee: Rohde and Schwarz GmbH and Co KG
Current assignee: Rohde and Schwarz GmbH and Co KG
Priority date: 1979-04-07
Filing date: 1980-04-03
Publication date: 1980-12-10
Also published as: DE2914143A1; US4325023A; FR2453415A1; GB2048499B; FR2453415B1; DE2914143C2

Description

1 GB 2 048 499 A 1

SPECIFICATION

A device for displaying a signal at a selected frequency in a band This invention relates to a device for displaying on a cathode ray tube screen, a signal at a selected frequency in a band, and one object is to provide a particularly simple device enabling the user to select whether to be able to analyse an input signal over the whole frequency band, orto pnerform a time analysis of a signal at a particular frequency in the band. There is preferably also means for giving a mark on the screen corresponding to the particular frequency.

According to the present invention, in a device for displaying on a cathode ray tube screen, a signal at a selected frequency in a band, a sweep voltage generated for the cathode ray tube time base is supplied, in one position of a switch, as the voltage 85 controlling the output frequency of a voltage controlled local oscillator used for heterodyning input signals to display signals at different frequen cies in the band on the screen, and in another position of the switch the local oscillator and its control are part of a phase-locked loop for setting the selected frequency, the device including means for storing the oscillator control voltage and a compara tor of the sweep voltage and the stored voltage arranged during the ramp of the sweep voltage to produce a frequency mark on the screen correspond ing to the selected frequency.

Thus, in the other position of the switch, the phase-locked loop serves to set the local oscillator frequency at a frequency corresponding to the selected frequency, and the control voltage for the local oscillator which sets that frequency can be stored for subsequent use by the comparator in setting the frequency mark on the screen.

During frequency analysis over the full frequency band, the frequency mark will indicate the particular selected frequency corresponding to the setting of the phase-locked loop.

The phase-locked loop preferably includes a com- parator of the phase of a reference frequency signal with the phase of the local oscillator output signal as modified in accordance with the setting of the loop, for example as divided by a preset divisor.

The use of the sweep voltage for the cathode ray tube time base as the voltage controlling the output frequency of the local oscillator causes the display to sweep through the frequency band in synchronisation with the time base on the horizontal axis of the tube, and the frequency mark will indicate on the screen the response at the particular frequency set in the phase-locked loop.

The device lends itself to ready transferto analysis over a time period of a selected frequency, because the local oscillator can be held at a constant output frequency with the phase-locked loop continually closed instead of using the sweep voltage to cause it to scan through the frequency band in each cycle. It is merely necessary to have the switch in its said other position. However, the selected frequency is still indicated by the frequency mark.

The switch may be arranged automatically to be in one position during the ramp of the sweep voltage, and in the ot her position during the return periods, or alternatively, it may be arranged automatically to be in the other position during every n th ramp and to be in the first position in the other ramps, or again it may be controlled in response to means for setting the selected frequency in the phase-locked loop so that the switch is switched to the other position only after an alteration in the setting of the selected frequency.

The invention may be carried into practice in various way, and one embodiment will now be described by way of example with referenceto the 80r accompanying drawing, of which the single figure is a schematic circuit diagram of a device constructed in accordance with the present invention.

The drawing is a circuit diagram of-a standard, panorama receiver having a receiver 1 in which the received signal at a frequency fa is converted bythe output at a frequency % from a tuning oscillator 2 into an output signal at a frequency fa which is suitable for analysis. The output signal fa can be supplied to the vertical deflection system of a cathode ray tube 6 by way of alternative modulator paths 4 and 5 one of which is selected by operation of a transfer switch 3. The horizontal deflection system of the cathode ray tube 6 is connected to a sweep generator 7 supplying a sawtooth sweep voltage.

The output frequency fu of the tuning oscillator 2 is tuned by the d,c. voltage applied to its input 8..

In a sweep operation, position v of an electronic switch 9 the sawtooth voltage of the sweep gener- ator 7 is supplied to the input 8 and the oscillators is repeatedly swept throught its frequency range. During frequency analysis, switch 3 is in its position F so thatthe vertical deflection signal is.supplied by way of the path 4, and the entire signal spectrum 10 of the swept frequency fa is displayed on the screen of the cathode ray tube 6 against the time base provided by the sweep voltage from the generator 7.

In the position r of the electronic transfer switch 9, the input 8 of the oscillator 2 is supplied not from the sweep generator 7, but by way of a line 11 from the output 12 of a frequency synthesizer of phase-locked loop which includes an adjustable frequency divider 13, a phase comparator 14 and a reference frequency source 15. The division factor of the frequency divider 13 is set by a digital counter 16, which is in turn set bya knob 17. If the oscillatoristo besetto an output frequency fu corresponding to the n-fold multiple of the reference frequency fr from the source 15, then the division factor of the frequency divider 131s set to n by way of the counter 16; the output frequency fu of the oscillator 2, divided by n, is compared with the reference frequency fr in the phase comparator 14. The comparator output vol tage at 12 adjusts the frequency of the oscillator 2 until when frequency and phase equality are achieved in the comparator, the tuning voltage U at the input 8 produces the desired frequency fu=nfr. The tuning voltage U is directly proportional to, or related to, the desired output frequency f,, related to the reference frequency fr by the setting of the 2 GB 2 048 499 A 2 adjustment knob 17, and that voltage u is, in the switch position r, stored in a storage capacitor 18. When the oscillator 2 is again switched to sweep operation in the position v of the switch 9, the tuning voltage U stored in the capacitor 18 is compared in a voltage comparator 19 with the sawtooth voltage of the sweep voltage generator 7. When there is voltage equality, a keying pulse is generated at the output of the comparator 19 which generates an intensification pip 20 on the screen of the cath.ocle ray tube 6, for example, by means of an appropriate control of the cathode beam. This intensification pip 20 can be displaced along the time base, and hence along the frequency spectrum 10 by adjusting the tuning voltage u of the synthesizer by means of the knob 17.

Change-over of the electronic switch 9 between the positions v and r, can occur automatically in response to the sawtooth voltage of the generator 7, as indicated diagrammatically at 21. During the ramps of the sawtooth voltage, the switch assumes the position v, and during the return periods of the sawtooth voltage, the switch assumes the position r.

Thus, during the ramps the frequency spectrum 10 is displayed on the screen together with the frequency mark 20 at the frequency corresponding to the voltage U previously stored in the capacitor 8. During the sweep returns the synthesizer loop is closed and the control voltage U corresponding to the frequency mark position as set at 17 is generated and stored.

Alternatively, the switch 9 can be automatically controlled from the sawtooth voltage at 21 in such a mannerthat, for example, during every second, third, or n th, ramp, the switch 9 assumes the position r, i.e. the sweep operation is briefly interrupted during selected ramps while the comparison voltage U is obtained. This is particularly suitable at high sweep frequencies in which such a brief blanking of the individual ramp has little noticeable effect on the 105 display.

The generation of the comparison voltage U need not occur regularly; it could be arranged to occur only when there is a change at 17 in the frequency adjustment, as indicated by the diagrammatic connection 22 from the divider 16 to the switch 9. After a change of frequency, the switch 9 can be changed to position r, for example, at the next sawtooth ramp and the voltage U corresponding to the new frequen- cy setting is stored for use during the next sweep operation (swi ' tch position v).

The operating mode transfer switch 3 is similarly coupled to the transfer switch 9. If the switch 3 is changed from the switch position F for frequency analysis and display of the entire spectrum 10 on the 120 screen as described above, to the switch position Z for time analysis, the switch 9 is simultaneously switched to position r and held there. Then the oscillator 2 supplies a fixed frequency f,, - which corresponds to the frequency setting of the synth esizer - to the receiver 1 and the fixed voltage U is stored at 18. Then the individual signal 23 of the frequency band whose frequency corresponds to the position of the frequency mark 20 is displayed and can be analysed by use of a modulator 5 with a 130 special transfer function or by use of additional receivers.

A standard phase comparator 14 usually contains a corresponding storage capacitor at the output for the integration of the deviation voltages occurring in successive phase comparison cycles, so that an additional storage capacitor 18 could be eliminated. This also applied to synthesizers in which the phase comparison circuit 14 supplies a control voltage U corresponding to the most recently selected frequency setting, even if the control loop is interrupted (switch position v). Then, not only the capacitor 18 but also the portion of the switch 9 arranged between the former and the phase comparator 14 can be eliminated, so that the line 12 is continuously connected to the comparator 19.

Claims

1. A device for displaying on a cathode ray tube screen a signal at a selected frequency in a band, in which a sweep voltage generated for the cathode ray tube time base is supplied, in one position of a switch, as the voltage controlling the output frequen- cy of a voltage-controlled local oscillator used for heterodyning input signals to display signals at different frequencies in the band on the screen, and in which, in another position of the switch, the local oscillator and its control are part of a phase-locked loop for setting the selected frequency, the device including means for storing the oscillator control voltage, and a comparator of the sweep voltage and the stored voltage arranged during the ramp of the sweep voltage to produce a frequency mark on the screen corresponding to the selected frequency.

2. Adevice as claimed in Claim 1 in which the phase-locked loop includes a comparator of the phase of a reference frequency signal with the phase of the local oscillator output signal as modified in accordance with the setting of the loop.

3. A device as claimed in either of the preceding claims, in which the switch is arranged automatically to be in the one position during the ramp of the sweep voltage, and to be in the other position during the return periods of the sweep voltage.

4. Adevice as claimed in Claim 1 orClaim 2 in which the switch is arranged automatically to be in the other position during every nth ramp of the sweep voltage, and to be in the first position in the other ramps of the sweep voltage.

5. Adeviceas claimed in Claim 1 orClaim 2 in which the switch is controlled in response to means for setting the selected frequency in the phaselocked loop so that the switch is switched from the one position to the other position only after an alteration in the setting of the selected frequency.

6. A device as claimed in any of the preceding claims in which the switch is an electronic switch.

7. A device as claimed in any of the preceding claims in which the storing means includes a capacitor arranged to be connected into the phaselocked loop when the switch in its other position.

8. A device as claimed in any of the preceding claims including a selector capable of supplying the heterodyned signals for display on the cathode ray k 3 GB 2 048 499 A 3 tube screen, either as a frequency spectrum over the frequency band for frequency analysis, or as a selected frequency within the band for time analysis, and in which when time analysis is selected, the switch is automatically set into its other position with the phase-locked loop supplying the control voltage to the local oscillator and thus setting the selected frequency.

9. A device for displaying on a cathode ray tube screen a signal at a selected frequency in a band arranged substantially as herein specifically described with reference to the accompanying drawing.

Printed for Her Majesty's Stationery Office by Croydon Printing Company Limited, Croydon Surrey, 1980. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.